1. Field of the Invention
The present invention relates to a photodetector, and more particularly to a photodetector with a signal processing capability.
2. Description of the Related Art
As shown in FIG. 1 of the accompanying drawings, one conventional adder with a photodetector comprises a pair of photodiodes 401, 402 constructed of n-type epitaxial layers 413, 414 and a p-type substrate 405. and a current-to-voltage (I/V) converter 400 for converting a current signal obtained from optical signals 409, 410 applied respectively to the photodiodes 401, 402 into a voltage signal, the I/V converter 400 comprising an operational amplifier 403, a feedback resistor 411, and an output terminal 404. The photodiodes 401, 402 are electrically insulated from each other by a p-type high-concentration layer 407. and connected to the I/V converter 400 by a metallic interconnection 412 for adding the optical signals.
A conventional photodetector is shown in FIG. 2 of the accompanying drawings. The photodetector shown in FIG. 2 is disclosed in Japanese laid-open patent publication No. 56274/92. The photodetector has a photodiode (photoelectric transducer) comprising a p-type well layer 502 and an n-type impurity layer 503, and electrically separated from other elements by a p.sub.+ -type impurity layer 505. The photodetector itself has no electric signal processing capability.
FIG. 3 of the accompanying drawings shows a signal processing circuit with a photodetector disclosed in Japanese laid-open patent publication No. 280079/86. As shown in FIG. 3, the signal processing circuit has a four-segment photodiode 601 for producing current signals that are supplied to an FM amplifier 604, an operational amplifier 608, and an operational amplifier 612 which perform a signal processing operation on the supplied signals. The four-segment photodiode 601 itself has no electric signal processing capability.
A conventional applied circuit which incorporates a photodetector is illustrated in FIG. 4 of the accompanying drawings. As shown in FIG. 4, a current signal from a photodiode 701 is converted by an I/V converter 702 into a voltage signal whose voltage gain is adjusted by an automatic gain control (AGC) circuit 703 to produce an optimum voltage signal at an output terminal 802.
FIG. 5 of the accompanying drawings shows a specific circuit arrangement of the AGC 703 shown in FIG. 4. As shown in FIG. 5, the AGC circuit 703 comprises NPN transistors Q.sub.1, Q.sub.2, Q.sub.3, resistors R.sub.1, R.sub.2, R.sub.3, and a current supply V.sub.in. Collector currents flowing into the NPN transistors Q.sub.1, Q.sub.2 are adjusted by controlling a control voltage applied between the bases of the NPN transistors Q.sub.1, Q.sub.2 for thereby controlling the gain of the AGC 703.
If photodiodes are connected as in the four-segment photodiode 601 shown in FIG. 3, then since crosstalk characteristics thereof are responsible for degraded signals, it is desirable to suppress such crosstalk characteristics. According to the crosstalk characteristics, a signal charge generated by light applied to a photodiode in the vicinity of an adjacent photodiode is detected as a signal of the adjacent photodiode due to diffusion and drift.
The structure of a photodiode which is designed to reduce crosstalk characteristics is illustrated in FIG. 6 of the accompanying drawings (see Japanese laid-open patent publication No. 82767/93). As shown in FIG. 6, a p.sup.+ -type high-concentration layer 918 is disposed between photoelectric transducers 901 for absorbing dripwise holes that are generated by light applied in the vicinity of the photoelectric transducers 901. thereby suppressing crosstalk.
However, the above conventional photodetectors with a signal processing capability have been disadvantageous in that since the photodetector (photodiodes) and the signal processor (circuit) are separate elements, the area and hence cost of the chip required are large.